1. Field of the Invention
The present invention relates to a load cell of a type wherein a bridge circuit having strain gauges is formed in a predetermined pattern on a surface of a strain inducing element and also to a weighing apparatus using such load cell.
2. Description of the Prior Art
A load cell employed in the prior art weighing apparatus generally comprises a strain inducing element for generating strain in response to a load applied thereto, a flexible wiring sheet such as, for example, a flexible printed circuit substrate and supported by the strain inducing element, and strain gauges fixedly bonded to the strain inducing element and having lead lines soldered to lines forming the flexible sheet to form a bridge circuit for detection of the strain. An example of this is disclosed in, for example, the Japanese Examined Patent Publication No. 59-2332 published in 1984.
On the other hand, in order to make the load cell compact, an attempt is made to form a thin-film bridge circuit on one surface of the stain inducing element by the use of an etching technique. However, where the thin-film bridge circuit is formed by the use of the etching, the following problems have been found.
Resistance elements of the strain gauges forming the bridge circuit are generally made of material having a relatively high resistance such as, for example, a copper-nickel alloy whereas the lead lines of the strain gauges and the lines bundled in the flexible sheet are made of a low resistance material such as gold. Therefore, where they are formed by the use of the thin-film technology, they are required to be etched separately, rendering the manufacturing process to be complicated.
On the other hand, in order to simplify the manufacturing process, it may be contemplated to make both of the strain gauges and the wiring lines of the same material such as, for example, a copper-nickel alloy so that the both can be etched during one process step. However, the use of the same material for the strain gauges and the wiring lines brought about an increase in resistance of the wiring lines. Once this occurs, not only the strain gauges, but also the wiring lines become susceptible to displacement of the strain inducing element and an error may occur in an output from the bridge circuit, with the consequence that the load cell will no longer provide a highly precise, accurate weighing result.
Also, since the strain inducing element is apt to deform with change in temperature, a pattern of distribution of temperature in the strain inducing element must be taken into consideration when a highly precise, accurate weighing result is desired.
By way of example, if an object to be weighed which is low in temperature, such as, for example, a frozen food product, is placed on the weighing table, the low temperature of the object is transmitted to the strain inducing element through the weighing table and, therefore, the temperature of one of the strain gauges adjacent one end of the strain inducing element, that is coupled with the weighing table, becomes different from that of the other of the strain gauges adjacent the opposite end of the strain inducing element, thereby creating a temperature gradient between the left and right strain gauges. Accordingly, where the temperature-dependent coefficient of resistance of the strain gauges is not zero, a change in resistance corresponding to the temperature gradient develops between the left and right strain gauges, accompanied by change in output of the bridge circuit to such an extent as to be unable to provide the highly precise, accurate weighing result.
Also, the Young's modulus of elasticity of one end of the strain inducing element increases with a decrease in temperature, accompanied by reduction in quantity of strain which leads to a weighing error.